Amylase, the most abundant enzyme in human saliva, binds specifically and with high affinity to several species of commensal oral streptococci that are early colonizers of the salivacoated tooth and numerous in supragingival dental plaque. Interestingly, amylase-binding streptococcal (ABS) colonization is restricted to hosts with detectable salivary amylase activity suggesting that amylase interactions are involved in oral colonization by ABS with an evolutionary basis. We have identified several amylase-binding proteins (ABPs) [AbpA (20-kDa) and AbpB (82-kDa) of Streptococcus gordonii, and AbpC (36 kDa) of Streptococcus mitis]. The binding of amylase to these bacteria is dependent only on AbpA in S. gordonii (and likely its homologs in other species), and not on other ABPs (such as AbpB). In vitro studies found ABPs to play a role in bacterial adhesion and biofilm formation. Interestingly, AbpA defective S. gordonii mutants were able to colonize rat mouths as well as the parental strains, suggesting that additional bacterial factors are involved in colonization and survival in vivo. Over the past funding period we have shifted our focus from a search for an adhesive/biofilm function for ABPs, to other potentially novel functions for these proteins. Preliminary studies suggest that amylase-binding to oral streptococci elicits altered gene expression in the bacterial cell. Thus, the Specific Aims of this proposal seek to address the following hypotheses: Hypothesis 1: ABPs alter gene expression in ABS. Hypothesis 2: AbpA interacts with other streptococcal components to modulate bacterial gene expression. Hypothesis 3: ABPs of different streptococcal species are related and function through a common structurally conserved amylase-binding domain. The proposed studies could contribute innovative knowledge to the understanding not only of dental plaque biofilm determinants, but also the nature of novel hostbacterial protein-protein interactions that mediate heretofore unrecognized cell signaling pathways.